1. Technical Field
The present invention relates to an image forming apparatus and an image forming method each for use in a copier, facsimile, printer, or the like and particularly for effecting recording by ejecting a toner as a developer onto a recording sheet.
2. Background Art
As the capabilities of personal computers have increased in recent years, a large quantity of documents are handled in offices, while a copier, a facsimile, and a printer having high processing abilities have been used widely due to the advancement of network technology. On the other hand, color documents tend to increase because of rapid prevalence of an inkjet printer and the like. However, an engine capable of outputting monochrome and color documents which are satisfactory both in printing speed and in image quality is still under development, so that the advent of such an engine as mentioned above is expected in this field.
As a conventional image forming apparatus, there is one using a direct marking method wherein an image is formed directly on paper. For example, the image forming apparatus disclosed in Japanese Unexamined Patent Publication No. SHO 63-136058 is aproposal of an apparatus of toner ejection type which is one of direct marking methods.
Below, an example of the foregoing conventional image forming apparatus will be described with reference to the appended drawings.
FIG. 17 is a sectional view showing a schematic structure of the conventional image forming apparatus. In FIG. 17, a toner 105 as a developer is filled in a developer hopper 104, and the toner 105 is guided to the position of a toner layer regulating blade 103 in contact with a toner transport roller 101 by the rotating action of a toner supply roller 102 and of the toner transport roller 101.
As shown in FIG. 17, a flexible printed circuit 110 is disposed under the developer hopper 104 in such a manner as to cover an opening 104a. A backside electrode 111 is provided in opposing relation to the flexible printed circuit 110. A direct-current power source 112 is connected to the backside electrode 111. In the structure, a recording sheet 122 from a sheet supply roller 120 passes over the backside electrode 111 to be guided to a heat roller 123.
FIG. 18 is a detailed diagram showing, under magnification, a part of the flexible printed circuit 110 of FIG. 17. In FIG. 18, part (a) is a vertical sectional view of the flexible printed circuit 110 and part (b) is a plan view of the flexible printed circuit 110 when viewed from the toner transport roller 101. It is to be noted that the part (a) of FIG. 18 is a sectional view taken along the line A-Axe2x80x2 of the part (b) of FIG. 18.
As shown in the part (a) of FIG. 18, the flexible printed circuit 110 has image signal electrodes 115 provided on the top surface (surface opposed to the toner transport roller 101) of a flexible printed circuit base 110a. The image signal electrodes 115 are provided in such ring-shaped configurations as to surround the top-face openings of apertures 116 which are through holes formed in the flexible circuit base 110a, and are electrically connected to signal voltage control means 117.
As shown in the part (b) of FIG. 18, the flexible printed circuit 110 has the apertures 116 and the image signal electrodes 115 corresponding to the apertures 116 which are arranged in plural numbers along the width of the recording sheet 122 and is disposed such that a line drawing in a direction along the width of the recording sheet 22 is formed.
Next, a description will be given on the operation of the conventional image forming apparatus thus constituted.
As shown in FIG. 17, the toner 105 is supplied from the toner supply roller 102 to the toner transport roller 101 which is grounded and the toner 105 is formed into a thin layer having a uniform film thickness by the toner layer regulating blade 103. The thin-layer toner 105 thus formed is a non-magnetic material having an amount of charge of xe2x88x9210 xcexc/g and an average particle diameter of 8 xcexcm.
The toner layer formed on the outer circumferential surface of the toner transport roller 101 is transported to a proximate position at a distance of about 30 xcexcm from the image signal electrodes 115 (the part (a) of FIG. 18) of the flexible printed circuit 110. At this time, when a voltage of +300 V is applied to the image signal electrodes 115 by the image signal voltage control means 117, the toner 105 passes through the apertures 116 and jumps in the direction of the recording sheet 122. A voltage of +1000 V is applied to the backside electrode 111 and the toner 105 that has jumped is attracted to the backside electrode 111 to land on the recording sheet 122, thereby forming dots.
If the voltage to the image signal electrodes 115 is switched to 0 V, the jumping of the toner 105 from the toner transport roller 101 is suppressed and the landing of the toner 105 on the recording sheet 122 is inhibited, so that non-dot parts (blank) are formed. An image is recorded on the recording sheet 122 by controlling the voltage applied to the image signal electrodes depending on the positions of the apertures 116.
However, the conventional image forming apparatus as described above had the following problems.
If the voltage to the image signal electrodes 115 is switched to 0 V during the non-dot part (the blank) formation, the jumping of the toner 105 from the toner transport roller 101 is suppressed. However, the toner 105 jumping between the image signal electrodes 115 and the recording sheet 122 is scattered to land on the periphery of the dots formed on the recording sheet 122, so that the image formed on the recording sheet 122 is in a fogged state.
Moreover, in the conventional image forming apparatus required the same number of image signal voltage control means 117 for applying the voltage to the image signal electrodes 115 as the corresponding number of image signal electrodes 115. In the case where each of the image signal voltage control means 117 is constituted with a change-over switch, 2560 or more switches are necessary to control the flexible printed circuit 110 which covers, e.g., the transverse length (about 8.53 inches) of A4 size at a recording density of 300 dpi. If the recording density is 600 dpi, 5000 or more switches become necessary.
In developing an image forming apparatus of toner ejection type using the flexible printed circuit 110, therefore, it has been a significant challenge in this field to effect high-density recording with a minimum number of switches in terms of production cost. In the case of thus reducing the number of switches, however, an increase in the complexity of the wiring pattern of the flexible printed circuit 110 may lead to higher cost. To achieve reasonable cost, therefore, the number of switches has to be reduced, while a simpler wiring pattern is used.
It is an object of the present invention to provide an image forming apparatus and an image forming method which have solved the foregoing challenge. In accordance with this invention, there can be formed homogeneous dots free from fog at an image by optimizing a voltage applied to electrodes for an image signal. It is another object of the present invention to provide an image forming apparatus and an image forming method whereby a stable image resistant to environmental and secular changes is formed at reduced manufacturing cost.
Still another object of the present invention is to provide an image forming apparatus and an image forming method which enable the formation of dots high in density and smaller in diameter. Yet another object of the present invention is to provide an image forming apparatus and an image forming method which enable the modulation of dot density and the changing of dot diameter.
To attain the aforesaid objects, an image forming apparatus according to the present invention comprises:
a developer carrying member for carrying at least a charged developer in an image formation region;
a counter electrode disposed in opposing relation to the developer carrying member in the image formation region and supporting a recording member, a specified voltage being applied to the counter electrode;
an insulating base disposed between the developer carrying member and the counter electrode and having a plurality of openings;
image signal electrodes provided around the respective openings in the insulating base in opposing relation to the developer carrying member, an image signal for controlling the amount of developer supplied from the developer carrying member and passing through the openings being applied to the image signal electrodes; and
a control signal electrode provided on the insulating base in opposing relation to the counter electrode, a voltage always lower than the voltage applied to the image signal electrodes being applied to the control signal electrode when a developer of negative polarity is used and a voltage always higher than the voltage applied to the image signal electrodes being applied to the control signal electrode when a developer of positive polarity is used.
In the image forming apparatus of the present invention thus structured, the control signal electrode to which the voltage is applied is thus provided, so that an electric field for causing the toner to jump to the developer passage holes is stably generated even during dot formation and during non-dot formation. Accordingly, the toner jumping through the apertures between the image signal electrodes and the recording sheet is prevented from landing the periphery of dots on the recording sheet at the moment at which the voltage to the image signal electrodes is switched, so that the image forming apparatus of the present invention enables the formation of homogeneous dots free from fog at the formed image.
An image forming apparatus in another aspect of the present invention comprises:
a developer carrying member for carrying at least a charged developer in an image formation region;
a counter electrode disposed in opposing relation to the developer carrying member in the image formation region and supporting a recording member, a specified voltage being applied to the counter electrode;
an insulating base disposed between the developer carrying member and the counter electrode and having a plurality of openings;
image signal electrodes provided around the respective openings in the insulating base in opposing relation to the developer carrying member, an image signal for controlling the amount of developer supplied from the developer carrying member and passing through the openings being applied to the image signal electrodes; and
a control signal electrode provided on the insulating base in opposing relation to the counter electrode, a voltage synchronized with the voltage applied to the image signal electrodes being applied to the control signal electrode, a voltage always lower than the voltage applied to the image signal electrodes being applied to the control signal electrode when a developer of negative polarity is used and a voltage always higher than the voltage applied to the image signal electrodes being applied to the control signal electrode when a developer of positive polarity is used.
In the image forming apparatus of the present invention thus structured, there is provided the control signal electrode to which a voltage synchronized with the voltage applied to the image signal electrodes is applied, so that the ejection of a toner as the developer from the developer passage holes is controlled accurately and an image forming apparatus which forms an image having stable and excellent image quality is provided.
An image forming apparatus in another aspect of the present invention comprises:
a developer carrying member for carrying at least a charged developer in an image formation region;
a counter electrode disposed in opposing relation to the developer carrying member in the image formation region and supporting a recording member, a specified voltage being applied to the counter electrode;
an insulating base disposed between the developer carrying member and the counter electrode and having a plurality of openings;
image signal electrodes provided on the insulating base in opposing relation to the developer carrying member, the image signal electrodes being formed in a plurality of rows to surround the respective openings, the image signal electrodes in different rows being electrically connected to each other to form a plurality of groups, different voltages being applied to the image signal electrodes in different groups during dot formation and during non-dot formation; and
control signal electrodes provided on the insulating base in opposing relation to the counter electrode, the control signal electrodes forming groups corresponding to the individual rows of the openings, different voltages being applied to the control signal electrodes in different groups during dot formation and during non-dot formation.
In the image forming apparatus of the present invention thus structured, the divided control signal electrode is provided for each row of apertures in the insulating base, so that rational grouping is performed without complicating a circuit pattern placed on the insulating base and therefore an image forming apparatus of high quality is provided at low cost.
In an image forming apparatus in""still another aspect of the present invention, high-voltage power-source control means for controlling the voltage applied to the control signal electrodes applies a voltage always lower than the voltage applied to the image signal electrodes when a developer of negative polarity is used and applies a voltage always higher than the voltage applied to the image signal electrodes when a developer of positive polarity is used.
In the image forming apparatus of the present invention thus structured, the operation of ejecting a toner as the developer from the same group of developer passage holes is selected by switching the voltage to the control signal electrode and the voltage applied to the image signal electrodes for the same group of developer passage holes is controlled by the same high-voltage power source control means, so that the number of the high-voltage power source control means is reduced significantly and the image forming apparatus is provided at low cost.
An image forming apparatus in yet another aspect of the present invention comprises:
a developer carrying member for carrying at least charged particles in an image formation region;
a counter electrode disposed in opposing relation to the developer carrying member in the image formation region and supporting a recording member, a specified voltage being applied to the counter electrode;
an insulating base disposed between the developer carrying member and the counter electrode and having a plurality of openings;
image signal electrodes provided around the respective openings in the insulating base in opposing relation to the developer carrying member, an image signal for controlling the amount of developer supplied from the developer carrying member and passing through the openings being applied to the image signal electrodes;
a control signal electrode provided on the insulating base in opposing relation to the counter electrode and controlling the behavior of the developer that has passed through the openings;
image signal switching means for applying a continuous variable voltage to the image signal electrodes; and
control signal switching means for applying the continuous variable voltage to the control signal electrode.
The image forming apparatus of the present invention thus structured enables the formation of dots which are high in density and small in diameter, the modulation of dot density, and the changing of dot diameter.
An image forming method of the present invention comprises the steps of:
carrying at least a charged developer by using a developer carrying member in an image formation region;
placing a recording member on a counter electrode in opposing relation to the developer carrying member in the image formation region and applying a specified voltage to the counter electrode;
applying a specified image signal to image signal electrodes provided on an insulating base having a plurality of openings and disposed between the developer carrying member and the recording member such that the image signal electrodes surround the respective openings in opposing relation to the developer carrying member and thereby controlling the amount of developer supplied from the developer carrying member and passing through the openings;
applying a specified control signal to a control signal electrode provided on the insulating base in opposing relation to the counter electrode and thereby controlling the behavior of the developer that has passed through the openings; and
applying, to the control signal electrode, a voltage always lower than the voltage applied to the image signal electrodes when a developer of negative polarity is used and applying a voltage always higher than the voltage applied to the image signal electrodes when a developer of positive polarity is used.
In the image forming method of the present invention, respective electric fields are formed between the developer carrying member and the image signal electrode and between the control signal electrode and the counter electrode and the jumping of a toner as the developer is controlled with the combined force of the two electric fields, so that control is effected more easily than in accordance with the conventional image forming method and the electric fields to be combined with each other for causing the toner to jump to the developer passage holes are generated stably constantly even during dot formation and during non-dot formation. In accordance with the present invention, therefore, the toner jumping between the image signal electrodes and the control signal electrode at the moment at which the voltage to the image signal electrodes is switched is prevented from landing on the periphery of a dot formed on a recording sheet, so that excellent dot formation is performed.
An image forming method in another aspect of the present invention comprises the steps of:
carrying at least a charged developer by using a developer carrying member in an image formation region;
placing a recording member on a counter electrode in opposing relation to the developer carrying member in the image formation region and applying a specified voltage to the counter electrode;
applying a specified image signal to image signal electrodes provided on an insulating base having a plurality of openings and disposed between the developer carrying member and the recording member such that the image signal electrodes surround the respective openings in opposing relation to the developer carrying member and thereby controlling the amount of developer supplied from the developer carrying member and passing through the openings; and
applying a specified control signal to a control signal electrode provided on the insulating base in opposing relation to the counter electrode and thereby controlling the behavior of the developer that has passed through the openings,
the control signal applied to the control signal electrode being a repetitive signal synchronized with the image signal applied to the image signal electrodes and shifted in phase from the image signal.
The image forming apparatus of the present invention thus structured enables the formation of dots which are high in density and smaller in diameter, the modulation of dot density, and the changing of dot diameter.